1. Field of the Invention
The present invention relates to an arc welding system in which arc welding is carried out while a welding torch of a welding robot is weaved.
2. Description of the Related Art
When arc welding is carried out on a work, such as a thick plate, along a welding line by using a welding robot, a welding torch of the welding robot is weaved to the left and the right. To realize the arc welding with good quality in such a case, it is essential to detect a position deviation between the welding torch and the welding line on the work, and to perform the arc tracking while correcting the position of the welding torch such that the welding torch tracks the welding line (e.g., Japanese Unexamined Patent Application Publication No. 61-095780 and No. 2008-093670).
The arc tracking will be described in brief below in connection with an illustration of a known arc welding system 9 by referring to FIG. 9. As illustrated in FIG. 9, the arc welding system 9 includes a welding robot 91, a welding power supply 92, and a controller 93.
The welding robot 91 is, e.g., an arc welding robot such as a 6-axis vertical articulated robot. A welding torch 91a is mounted to an arm fore end of the welding robot 91. Further, a welding wire 91b is fed to the welding torch 91a by a welding-wire feed motor (not shown).
The welding power supply 92 is a power supply for supplying a welding current to the welding wire 91b. Further, the welding power supply 92 communicates with the controller 93 via an analog communication path An by using an analog signal. The term “analog signal” used in the present invention means a signal obtained by continuously converting data to a voltage level.
The controller 93 is a control device for controlling the welding robot 91 and the welding power supply 92. Further, the controller 93 includes a tracking control unit 93a to perform the arc tracking.
With such a configuration, the arc welding system 9 carries out arc welding on a work W in a state that arc discharge is generated between the work W and the welding wire 91b by the welding current supplied from the welding power supply 92, while the welding torch 91a is moved over the welding line by the welding robot 91. During the welding, the tracking control unit 93a in the arc welding system 9 detects a position deviation of the work W and corrects the fore end position of the welding torch 91a for the purpose of arc tracking.
Recently, users of the above-described arc welding system have strongly demanded makers of welding robots to realize more upgrading of practical techniques in the arc welding and more sophisticated functions of arc welding machines. When the makers of welding robots try to satisfy those demands, it is indispensable to accurately communicate a larger amount of data between a robot controller and a welding power supply.
In trying to communicate a larger amount of information in the known arc welding system 9 that performs communication using analog signals as illustrated in FIG. 9, however, the communication path An for the analog signal needs to be increased in number whenever a new type of signal (e.g., a universal signal for detection of abnormality) is added. This gives rise to the problem that cable wiring is complicated and reliability of the communication is reduced.
Communication using digital signals is inevitable to solve the above-mentioned problem. However, the use of the digital communication gives rise to another problem that a delay is caused in the communication and accuracy in the arc tracking is reduced in comparison with the communication using analog signals.
To cope with those problems, the invention described in Japanese Unexamined Patent Application Publication No. 2003-103366 enables real-time performance to be improved by interconnecting various control units through a parallel bus system and by performing communication using digital signals. However, the digital communication with the parallel bus connection greatly increases the number of communication paths and hence reduces reliability of the communication. Further, if missing of signal data (e.g., missing of bits) occurs, a complicated recovery process and an abnormality detection process are required. In addition, because the parallel bus system is susceptible to noises, a cable length has to be held relatively short, e.g., several meters or less. System layout, etc. are thereby restricted in designing the configuration of the arc welding system. Particularly, system layout, etc. are very restricted when the welding is carried out by using a plurality of power supplies connected to each other.
An object of the present invention is to solve the problems described above and to provide an arc welding system in which digital communication can be performed with increased communication reliability, system layout can be designed at relatively high flexibility, and accurate arc tracking can be ensured.